FIG. 1 is a block diagram of a prior art circuit which comprises two-wire, electromechanical, thermal switch 200, which switch 200 is itself comprised of temperature sensing probe 210. Two-wire, electromechanical, thermal switch 200 turns electrical load 220 on or off whenever a specified temperature is detected by temperature sensing probe 210. As illustrated in FIG. 1, two-wire, electromechanical, thermal switch 200 comprises metallic contacts 215 for use in switching and switch 200 comprises mechanical components (not shown) for transforming a temperature difference into a change in a force-distance-product in temperature sensing probe 210--in this case temperature sensing probe 210 is a thermo-mechanical temperature sensing probe. Whenever switch 200 is on and electrical load 220 is being energized by power source 240, the voltage across terminals 217 and 219 of switch 200 is just the electrical drop across metallic contacts 215 and the connections thereto.
A switch like switch 200 of FIG. 1 is undesirable because metallic contacts 215: (a) erode from use; (b) generate an arc when switching occurs; and (c) often corrode from disuse. As a result of this, a solid state, electronic switch is an attractive alternative because, unlike a metallic contact switch, a solid state, electronic switch: (a) is indifferent to the number of operations it performs; (b) provides arcless switching; and (c) is indifferent to the interval between switching operations.
One method of fabricating a solid state, electronic switch is to use a thermo-mechanical temperature sensing probe and to use a metallic contact to turn a solid state output device such as, for example, a transistor, on and off. Such a switch is undesirable, however, because such a switch would still be limited, in the manner described above, by the use of metallic contacts and by the use of a thermo-mechanical temperature sensing probe. These limitations may be overcome by developing means for performing temperature sensing functions and means for performing output-control functions which are fabricated using electronic components. This is not as simple as it sounds, however, because, when the electrical load is switched on, the voltage drop across the terminals of a typical two-wire, thermal switch that has a solid state output device is in the range of 0.5 volts to about 2.0 volts--the specific value depends on the load and the operating characteristics of the solid state output device used. In practice, the voltage drop across the switch is deliberately designed to be small or negligible when the switch is on to minimize power dissipation in the switch. A problem arises in this regard because this small voltage drop is not adequate to power electronic circuits in the switch which are used to sense temperature and circuits in the switch which are used to provide electronic output-control. As a result, a voltage source is required to provide power to electronic components which would be used in such a thermal switch application.
One method of solving the need to provide power is to use a battery. However, this solution is undesirable because maintenance is required whenever a battery is used. An alternative method of solving this need to provide power is to add a third wire to the switch to supply the switch directly with power from the power source which is connected to the load. However, this solution is undesirable in an application in which an all-electronic switch is to replace a two-wire, electromechanical switch without subsidiary changes.
As a result, there is a need in the art for a two-wire, electronic switch which performs sensing functions, measurement functions, and output-control functions for an electrical load without receiving power from a power source, external or internal, other than the power source for the electrical load that is controlled by the switch, and for a two-wire, electronic switch which can do all of that notwithstanding whether the switch is on or off.